(1) Field of the Invention
The present invention relates to a surface acoustic wave device that utilizes a plurality of acoustic surface wave guides and periodic arrays of reflectors that receive an input signal and generate an output signal with only a single vibration mode acoustic wave generated in the substrate.
(2) Background of the Related Art
In the 1975 proceedings of the Ultrasonic Symposium, on pages 307-310, Staples et al discussed a bulk-wave resonator with trapped energy modes. This article described the SAW resonator model as consisting of a region of slow wave velocity (the reflectors and transducers) surrounded by regions of faster wave velocity, giving rise to wave guide cut-off phenomena, in other words, energy trapping. The extent to which the waves are trapped and the number of distinct modes trapped depend upon the velocity difference and the width of the trap region. FIG. 3 on page 308 of the Staples et al article illustrates the expanded response of a SAW resonator with shorted reflecting stripes. The spurious modes are indicated by arrows on the high frequency side of the response. These spurious modes are trapped energy modes and are sometimes referred to as inharmonic spectra. To verify that this was the case, resonators were constructed and tested and the results recorded as illustrated in FIG. 4 on page 308 of the Staples et al article in the form of dispersion curves. The dispersion curves are obtained by fitting the transcendental equation (symmetric case) associated with a thin-film wave guide to the obtained data points. The frequency of each mode, including the fundamental, shifts up in frequency as the width of the resonator is decreased. Furthermore, the number of modes present or trapped also decreases until at 10 wavelengths only one symmetric mode remained.
At the same 1975 proceedings of the Ultrasonic Symposium, Tiersten et al in an article entitled "Guided Acoustic Surface Wave Filters", pages 293-4, discussed a number of filter structures utilizing acoustic surface wave guides and periodic arrays reflecting strips to form multimoded resonant configurations. This article indicated that the devices described are analogous to the monolithic crystal filter in that an array of acoustically coupled resonators is obtained from a structure placed on a single substrate. The coupling between resonators is described in the article in terms trapped energy modes of the surface wave guiding structures. This article points out that if two interdigital transducers are placed in parallel on a piezoelectric substrate, between two arrays of reflecting strips, as shown in FIG. 1 on page 294 of the article, and both the reflecting arrays and the interdigital transducers are partially encased in two thin strips of a "slow" insulating material deposited on the surface of the piezoelectric substrate, a two-pole bandpass filter is created with a bandwidth controllable by the spacing between the electrodes in the reflecting array, the width of the overlapping portion of the electrodes in each of the transducer structures, the thickness of each identical strip and the spacing between the strips which act as thin-film wave guides. This article points out that in operation, the driving transducer excites, at the driving frequency, both the fundamental symmetric and asymmetric modes with respect to the center line of the structure to about the same amplitude. Since, at the driving frequency, the wavelength of each mode differs, and since both modes are reflected by arrays with the same spacing, the reflection of each mode will have different frequency characteristics. Consequently, the detecting circuit, which responds to the sum of the two modes, will experience a response as shown in FIG. 3 on page 294 where the magnitude of the passband ripple is determined by the resistances in the external circuitry. The structures described are in fact coupled resonator structures and may be thought of as surface wave analogs of the widely used bulk-wave monolithic crystal filter. As stated in the article, as in the case of other bandpass devices such as the monolithic crystal filter, these structures may be used not only as complete monolithic bandpass filters and as bandpass filter sections wherein two or more structures are connected in tandem to form higher order filters but also in various other ways, including several types of frequency discriminators or demodulators and band-reject filters.
Tiersten et al then obtained U.S. Pat. No. 4,060,777 dated Nov. 29, 1977 disclosing the resonator and resonator-type wave filter formed by a piezoelectric body on which acoustic surface waves are guided and reflected and in which mode coupling in a direction transverse to the surface wave propagation is utilized. This patent discloses a surface wave device formed on a piezoelectric body in which the transverse mode structure of the device is controlled by controlling the mass loading of reflector and transducer elements of the structure. At the resonant frequency of the device, a standing wave is trapped or largely confined in the transverse direction by the wave-guiding action of the reflectors and the transducer. Each of the electrode fingers of the transducer has a particular length and is placed so as to have a maximum interaction with the standing wave. If the trapping in the transverse direction is excessive, more than one resonance may exist. The resonance lowest in frequency is the fundamental symmetric mode. The next higher mode is the fundamental asymmetric mode. Successively higher modes are alternately symmetric and asymmetric with respect to the longitudinal center line of the resonator structure. The resonance curve of a resonator having excessive trapping in the transverse direction is shown in FIG. 2A of U.S. Pat. No. 4,060,777 which is incorporated herein by reference in its entirety. The shape of the fundamental symmetric mode of the standing wave in the transverse direction is shown in FIG. 3A of U.S. Pat. No. 4,060,777. By properly dimensioning the resonator, the number of transverse modes which are trapped and their frequency spacing may be controlled by controlling the width of the electrode fingers and/or the thickness and/or the mass of the reflecting strips. By suitable choice of material it is possible to trap only the fundamental symmetric transverse mode of the fundamental symmetric and asymmetric transverse modes.
FIG. 4 of U.S. Pat. No. 4,060,777 shows a filter structure wherein two interdigital transducers are placed on a piezoelectric substrate between two arrays of reflecting strips. A source of energy is connected to one transducer which is the input of the device and a load is connected to the other transducer which is the device output. By properly selecting the width of the reflectors and/or the thickness and/or mass of the reflecting strips, both the symmetric and the asymmetric fundamental transverse modes may be excited.
In U.S. Pat. No. Re. 33,957 issued Jun. 9, 1992, a high-frequency narrow bandpass multimode filter is constructed with resonators closely disposed transversely to each other on the single piezoelectric substrate to generate different vibration modes of different resonant frequencies. As set forth in that patent, when two SAW resonators are transversely disposed on a piezoelectric substrate in a parallel and closely opposed relation and when the resonators are excited, the resultant SAWs are acoustically coupled to each other and the resonators generate two vibration modes, that is, the symmetrical mode and the asymmetrical mode.
In these prior art devices, for a given bandwidth of the device there is low coupling and a high impedance. When the beamwidth of the device increases, the bandwidth decreases and the coupling increases. When the beamwidth decreases, the bandwidth increases and the coupling decreases. Thus, a significant disadvantage exists with the prior art. For a given bandwidth, if one wishes to increase the coupling, the track width, or beamwidth, must be increased. However, when the track width is increased, the bandwidth decreases.